Light emitting diode (LED) based luminaires often comprise modules including printed circuit boards (PCBs) on which the LEDs are mounted. The LEDs are typically arranged in an array or pattern and mounted on the PCB, often termed PCB assembly (PCBA), which is appropriate for the luminaire in which they are to be used. Whilst each LED comprises a light source with a primary optical element or lens, secondary optical elements are often required to direct the light produced by these light sources in directions appropriate to the particular implementation in a luminaire. Lenses are typically used as the secondary optical elements for directing the light.
In current applications, most secondary lenses are made out of rather rigid materials, such as polymethylmethacrylate (PMMA) or polycarbonate, and are mounted on the PCBA by the use of screws or rivets which provide precise positioning of the secondary lenses with respect to the associated LEDs.
Unfortunately, the use of PMMA or polycarbonate lenses as secondary lenses tends to limit the operating temperature of the PCBA. Conventional plastic materials such as PMMA and polycarbonate cannot be exposed to temperatures exceeding 85° C. without long term deterioration, for example, due to thermo-oxidation. The LEDs and PCBAs, particularly when they comprise metal core PCBs (MCPCBs), are more resistant to higher temperatures, and, the material from which the secondary lenses are most often made limits the maximum operable temperature of the assembly. This limitation in temperature necessitates the provision of good dissipation techniques on the luminaire to keep the temperatures under this limit. This requires costly heat dissipating structures, for example, heat sinks which increase the amount of metal, for example, aluminium, required in the manufacture of the luminaire housing or body in which the LEDs are mounted.
Silicone is now being implemented in more and more applications as secondary lenses for use with PBCAs as it can operate at higher temperatures than PMMA and polycarbonate without significant degradation of the material, whilst providing optical properties that remain consistent over extended periods of time. Silicone is a proven material for use in encapsulated LED primary lenses due to its ability to operate at higher temperatures.
However, when implementing silicone as a single moulded lens array, it becomes difficult to mount the array precisely on the PCBA and to maintain precise alignment between the secondary lenses and the LEDs. This is due to silicone itself being a soft material and the moulded lens array is flexible. More particularly, due to the relative softness and flexibility of the material, it is no longer possible to use conventional mounting techniques, for example, using screws, rivets, etc.
Furthermore, the goal of achieving higher operating temperatures generates even more severe constraints due to different thermal characteristics of the materials from which the secondary lenses and PCBA are made. These characteristics need to be taken into account to provide a reliable mounting of the lenses whilst also maintaining correct lens alignment with respect to the LEDs during temperature changes.